1. Field of the invention
The present invention relates to a multiplexer, and more particularly to a multiplexer which can select an signal terminal or terminals to which a signal is to be inputted.
2. Description of Prior Art
In general, a single-chip microcomputer is equipped with a number of input terminals for receiving signals and a built-in multiplexer for alternatively selecting one of the input terminals. The signal at a selected signal input terminal is inputted to an internal circuit.
The known single-chip microcomputer will be described by reference to FIG. 1, which is a circuit diagram thereof:
The single-chip microcomputer M is provided with signal input terminals AN.sub.0, AN.sub.1 . . . AN.sub.n-2 and AN.sub.n-1 each of which is respectively connected to one terminals of switching elements S.sub.0 to S.sub.n-1. The other terminals thereof are connected to an analog/digital converter (A/D converter) 1 at one point as shown in FIG. 1. In this way the switching elements S.sub.0 to S.sub.n-1 constitute a multiplexer.
The A/D converter 1 converts an analog signal at any one of the input terminals AN.sub.0 to AN.sub.n-1 which is selected by turning on or off the switching elements S.sub.0 to S.sub.n-1. The A/D converter 1 performs in a high accuracy A/D conversion upon the input signal having an optimum potential amplitude predetermined by a source voltage and other factors.
In FIG. 1, in a case where an analog signal to be converted into digital signal (hereinafter referred to as "target analog signal") is inputted to any one of the terminals AN.sub.0 to AN.sub.n-1 in the microcomputer M. If each of the target analog signals inputted to input terminals AN.sub.1 and AN.sub.n-2 has a smaller than the optimum potential amplitude, the smaller is amplified to have an optimum value by means of analog amplifiers 51 and 52 connected to the input terminals AN.sub.1 and AN.sub.n-2 as external circuit elements, by inputting analog signals thereto, as shown in FIG. 2. The analog amplifiers 51 and 52 are installed at the expense of the users. In this way all analog signals inputted to the input terminals AN.sub.0, AN.sub.1 . . . AN.sub.n-2 and AN.sub.n-1 can be inputted to the analog/digital converter 1 in an optimum potential amplitude.
The addition of the extra analog amplifiers to each of the input terminals increases the production cost. In addition, the amplitude of each analog amplifier must be precisely adjusted so as to have an optimum in accordance with the amplitude of the analog signal. This precise adjustment is costly. Users of single-chip microcomputers have to bear all expenses involved in equipping the analog amplifiers and adjusting the amplitude thereof.
In order to save the costs on the adjustment of amplitude of the analog amplifiers added by users, there is a proposal for an arrangement shown in FIG. 3 which includes an analog amplifier 5 connected to the input terminal AN.sub.1 alone, and a multiplexer 11 which is externally connected to the analog amplifier 5, selects an analog signal having a relatively small amplitude and inputs it to the analog amplifier 5.
This proposed arrangement is advantageous in eliminating the necessity of elaborate adjustment but disadvantageously it requires a multiplexer as an external circuit element. Therefore, the users have to control both multiplexers--an external multiplexer and an internal multiplexer of the single-chip microcomputer. As a result, the selection of a target analog signal becomes complicated. It is difficult to reduce the production costs of a single-chip microcomputer-based equipment.